Add Crabbox with E2B use case docs

[vsokolovskii]

Summary

• Add a focused Crabbox with E2B use-case page
• Add the Crabbox SVG icon and wire it into the page frontmatter
• Add the page to the Use cases navigation

Validation

• npx mintlify@latest broken-links
• Bash syntax check for the page command snippets
• Local Crabbox E2B warmup/status/stop flow against a fake E2B control API
• go test ./internal/providers/e2b in the sibling Crabbox repo